Marine engines are used in a variety of different types of vessels ranging in size and application from small recreational runabouts to large ocean-going vessels. New marine engines must meet increasingly stringent emissions requirements, yet these engines continue to emit significant amounts of pollution, which contribute to serious public health problems. Examples of said pollution include oxides of sulfur, oxides of nitrogen, hydrocarbons, heavy metals, CO, CO2, and particulate matter, all of which are harmful to humans, as well as the environment. Recent environmental regulations have decreased the allowable levels of sulfur in marine fuel and allowable amounts of pollutant emissions from marine engines, thereby creating significant environmental and public health benefits by reducing pollution from marine engines. Thus, there is a need for more efficient, environmentally friendly marine engine systems.
Marine shipping, however, is an industry that demands profits. The marine shipping industry employs hundreds of thousands of workers globally and generates approximately $400 billion in revenue globally. The profit enjoyed by marine shipping companies depends on the efficiency with which their operations are carried out at sea. In light of high fuel and operating costs of vessels, ship owners and operators are employing extra measures to maintain desired profit levels. Since the energy efficiency of a ship depends mainly on the propulsion and auxiliary power plant of the ship, much of the focus on energy efficiency has been directed to marine engines. Increased energy efficiency also results in reduced fuel consumption which results in less emissions. Thus, there is a need for more efficient marine engines.
The prior art of marine vessel engine exhaust gas scrubbing is well represented and summarized by U.S. Pat. No. 8,038,774B2; U.S. Pat. No. 8,500,893; U.S. Patent Pub. No. 20140248201; and U.S. Patent Pub. No. 20100230506. Gas scrubbing occurs when large quantities of seawater or an alkali reagent solution are sprayed into the exhaust gas. Heat energy recovery, if any, uses an internal gas-liquid heat exchanger operated at gas temperatures well above the exhaust gas condensable gas dew point to prevent corrosion, or by using conventional liquid-liquid heat exchangers to cool process liquids, e.g., collected spray and any process condensate. The heat energy recovered, if any, is used internal to the scrubbing process, e.g., for visible exhaust gas plume reheat mitigation or to produce fresh water for internal scrubber use. Net usable energy use by the ship with prior art scrubbers increases 1.0%-3.5% of the engine fuel energy use because of the scrubber energy use and low offsetting energy recovery. This, in turn, increases fuel consumption and paradoxically increases emissions of pollutant un-scrubbed gases such as CO and CO2.
The advantages and benefits of hot gas heat recovery and scrubbing with condensing heat exchangers are well documented by U.S. Pat. No. 5,368,096, U.S. Pat. No. 5,510,087, U.S. Pat. No. 5,534,230, and others. Instead of avoiding heat recovery at exhaust gas temperatures near or at the dew point of condensable exhaust gases, condensation and the recovery of the latent heat of condensation, as well as sensible heat, is facilitated. This allows significant usable energy recovery, typically well in excess of the energy used by as much as 10%-15% of the fuel energy value, and an increase of overall fuel efficiency and potential decrease in CO and CO2 emissions. Additionally, the condensing action provides a significant gas scrubbing benefit. Addition of alkali reagent sprays as a secondary scrubbing agent further improves exhaust gas quality.
A major marine vessel engine manufacturer states that marine vessel engine exhaust gas contains 25% of the heat energy of the fuel consumed and a large amount of pollutants. A portion of that energy, e.g., 10%, can be recovered for use with high efficiency exhaust gas boilers. Prior art scrubbers reduce certain emissions but diminish net energy efficiency and increase other pollutants.
Many of the currently available marine engine systems increase efficiency of the marine engine system in a way that increases the cost of the system to an untenable price or that do not meet stringent governmental environmental standards. Further, some of the available systems that are more economical in price and/or more environmentally friendly, do not adequately recover heat waste energy from marine engine systems and therefore do not adequately increase the energy efficiency of the system. Therefore, what is needed is a system and method for improving upon the problems with the prior art, and more particularly for a more efficient and cost-effective way to increase the efficiency of marine engine systems in a way that is also environmentally friendly.